Alkyl aromatic compounds, such as cumene and ethylbenzene, are often produced by reaction of alkylating alkylatable aromatic compounds with an alkylating agent. For example, cumene (or isopropylbenzene) is industrially produced by alkylation of benzene with propylene. Alkylation is an exothermic reaction. Cooling is needed to manage the temperature increase in the reactor. In the existing cumene production technology, the total reactor effluent containing cumene is cooled, recycled and re-introduced into the first alkylation reaction zone to manage temperature rise from the exothermic reaction of benzene with propylene. The recycled cumene, however, provides an opportunity for it to be further alkylated to undesired byproducts, for example, di-isopropylbenzene and tri-isopropylbenzene.
Many attempts have been made in the art to manage the temperature rise during the alkylation of benzene with C2 to C4 olefins.
U.S. Pat. No. 6,008,422 discloses a process for producing alkylaromatics using a multibed alkylation reaction zone. The alkylation reaction zone effluent is divided into three portions, the first being recirculated to the inlet of the alkylation reaction zone, the second being cooled and recirculated to one or more other beds in the alkylation reaction zone, and the third being passed to a product recovery zone where the alkylaromatic compound is recovered.
U.S. Patent Application Publication No. 2008-021252 discloses a process for the production of alkylated aromatic compounds, in which aromatic compounds from each of at least the first and second catalytic alkylation reaction zones are contacted with a cooling means for re-condensing at least a portion of the aromatic compounds vaporized from the exothermic heat of reaction of the alkylation process.
U.S. Pat. No. 5,336,821 discloses a process for the alkylation of aromatic hydrocarbons such as cumene and ethylbenzene, in which a portion of the effluent stream from an alkylation reactor passes through an indirect heat exchanger to transfer heat to a flashed stream containing the product aromatic hydrocarbons, and the heat exchanger recovers the exothermic heat of the reaction from the effluent stream for use elsewhere in the process.
U.S. Pat. No. 6,835,862 discloses a process for the alkylation of aromatics with olefins using a solid catalyst, wherein the olefin ratio and/or the maximum olefin concentration in the alkylation catalyst bed is maintained less than an upper limit.
U.S. Pat. No. 5,900,518 discloses a process for heat integration in an alkylation/transalkylation process, in which the alkylation effluent passes into the first separation zone which is operated to produce a lower boiling fraction comprising the aromatic substrate, which may be recycled to the alkylation reaction zone, and a higher boiling fraction comprising a mixture of monoalkylated aromatics and polyalkylated aromatics.
U.S. Patent Application Publication No. 2007-02657481 describes a process for producing an alkylaromatic compound in a multistage reaction system comprising at least first and second series-connected alkylation reaction zones each containing an alkylation catalyst. A first feed comprising an alkylatable aromatic compound and a second feed comprising an alkene and one or more alkanes are introduced into said first alkylation reaction zone. The first alkylation reaction zone is operated under conditions of temperature and pressure effective to cause alkylation of the aromatic compound with the alkene in the presence of the alkylation catalyst, the temperature and pressure being such that the aromatic compound is partly in the vapor phase and partly in the liquid phase. An effluent comprising the alkylaromatic compound, unreacted alkylatable aromatic compound, any unreacted alkene and the alkane is withdrawn from the first alkylation reaction zone and then supplied to the second alkylation reaction zone without removal of the alkane. The operating conditions in each of said first and second reaction zones are such that the ratio of the volume of liquid to the volume of vapor in each zone is from about 0.1 to about 10.
U.S. Pat. No. 6,995,295 discloses a process for producing an alkylaromatic compound by reacting an alkylatable aromatic compound with a feed comprising an alkene and an alkane in a multistage reaction system comprising at least first and second series-connected alkylation reaction zones each containing an alkylation catalyst. At least the first alkylation reaction zone is operated under conditions of temperature and pressure effective to cause alkylation of the aromatic compound with the alkene in the presence of the alkylation catalyst, the temperature and pressure being such that the aromatic compound is partly in the vapor phase and partly in the liquid phase. An effluent comprising the alkylaromatic compound, unreacted alkylatable aromatic compound, any unreacted alkene and the alkane is withdrawn from the first alkylation reaction zone and at least part of the alkane is removed from the effluent to produce an alkane-depleted effluent. The alkane-depleted effluent is then supplied to the second alkylation reaction zone.
U.S. Pat. No. 7,645,913 discloses a process for producing an alkylaromatic compound in a multistage reaction system comprising at least first and second series-connected alkylation reaction zones. In this process, the alkylation catalyst in the first alkylation reaction zone, which may be a reactive guard bed, has more acid sites per unit volume of catalyst than the alkylation catalyst in the second reaction zone.
In a process for the production of monoalkyl aromatic compounds, for example, cumene by alkylation of benzene with propylene, the byproduct, di-isopropyl benzene is formed by the subsequent alkylation of cumene:H6C3+H6C6→H12C9 H6C3+H12C9→H18C12 However the process is exothermic and usually comprises a step of recycling at least a portion of the reactor effluent to cooler and to the reactor inlet to dilute the feed and reduce the temperature rise from alkylation. (See FIG. 1.) A recycle ratio is defined herein as a weight ratio of the recycled portion of the reactor effluent to the non-recycled portion of the reactor effluent. In such a process, the inventor realizes that, as the recycle ratio increases, the average concentration of monoalkyl aromatic compound at the reactor inlet increases, thus providing opportunity for further reaction of the monoalkyl compounds to polyalkyl compounds. FIG. 2(a)-(c) shows in units of mole fraction that as the recycle ratio is increased, the formation of the diisopropylbenzene (DiPB) byproduct also increases. As a result, the undesirable byproduct selectivity of diisopropylbenzene (defined as the ratio of dialkylbenzene divided by cumene) increases with increasing recycle ratio, as shown in FIG. 3. Thus, FIG. 3 indicates that byproduct selectivity can be lowered by operating at lower recycle ratios.
Therefore, there is a need to devise a process for production of monoalkyl aromatic compounds by alkylation of alkylatable aromatic compounds with an alkylating agent with high selectivity to desired monoalkyl aromatic and low formation of byproducts such as polyalkyl aromatic compounds.